Understanding the structural and chemical evolution of layered potassium titanates for sodium ion batteries

Layered potassium titanates are promising anodes for sodium ion batteries, owing to their superior cycling stability, huge abundance, and environmental benignity. The structural and chemical evolution mechanisms of layered potassium titanates are still not entirely clear, however, which is a huge obstacle to their practical application. Herein, we develop layered K₂Ti₄O₉ as a model anode of potassium titanates for SIBs and to investigate the corresponding structural and chemical evolution during cycling process. The results demonstrate that the crystal structure of K₂Ti₄O₉ is well maintained with the incorporation of sodium ions and a zero-strain characteristic is observed upon sodium insertion/extraction process. Furthermore, about half of the potassium ions dissolve out of the K₂Ti₄O₉ host structure, and equal amounts of sodium ions then occupy the vacancies created by the potassium ions during charging/discharging processes, eventually resulting in the formation of KNaTi₄O₉. Density functional theory calculations further confirm the possibility of the generation of KNaTi₄O₉ and indicate that the intercalated sodium ions are more likely to occupy the K2 Wyckoff sites.